This invention relates to an apparatus for measuring a reflection characteristic of a sample using an integrating sphere, which may be adopted in a spectral color measuring device.
Generally, measurement of a reflection characteristic of a sample is greatly affected by a configuration of an illuminator and a light receiving device (hereinafter, referred to as "geometric configuration"). Accordingly, almost all reflection property measuring devices such as spectral color measuring devices employ any one of the following geometric configurations which are recommended by the International Commission on Illumination (CIE: Commission Internationale de l'Eclairage).
45/0: the illuminator is so arranged as to illuminate the sample surface with light incident upon the sample surface at 45.degree., and the light receiving device is so arranged as to receive light reflected from the sample surface at 90.degree.;
0/45: the illuminator is so arranged as to illuminate the sample surface with light incident upon the sample surface at 90.degree., and the light receiving device is so arranged as to receive light reflected from the sample surface at 45.degree.;
d/0: the illuminator is so arranged as to illuminate the sample surface with diffused light, and the light receiving device is so arranged as to receive light reflected from the sample surface at 90.degree.; and
0/d: the illuminator is so arranged as to illuminate the sample surface with light incident upon the sample surface at 90.degree., and the light receiving device is so arranged as to receive diffused light.
Among the above configurations, d/8 type (combination of diffused-light-illuminator and +8.degree.-inclined-light-receiving-device), a variation of the d/O configuration, has been widely used because it can measure both a reflection characteristic of a specular component included reflection light (or SCI spectral reflection) and a reflection characteristic of a specular component excluded reflection light (or SCE spectral reflection). The SCI spectral reflection is unlikely to be influenced by the surface condition of the sample and hence has measurement stability, and the SCE spectral reflection is close to visual sense.
To measure a reflection characteristic of SCI spectral reflection (hereinafter, merely referred to as "SCI reflection characteristic") and a reflection characteristic of SCE spectral reflection (hereinafter, merely referred to as "SCE reflection characteristic") in the d/8 type geometric configuration, there has been primarily used an integrating sphere provided with a mechanically openable trap member in an inner wall thereof. The inner wall of the trap member functions as a light source for specular reflection, and the SCI reflection characteristic is measured by closing the trap member and the SCE reflection characteristic is measured by opening the trap member.
Japanese Unexamined Patent Publication No. 9-61743 discloses a reflection characteristic measuring apparatus provided with an integrating sphere in which two kinds of illumination light with different light distributions illuminate a sample to measure a reflection characteristic of a sample without a mechanical device such as a trap member.
FIGS. 5 and 6 are schematic construction diagrams of the reflection characteristic measuring apparatus in the above publication.
FIG. 5 shows light distribution in an integrating sphere 100 when a first illuminator 110 is driven to emit light. Assuming that the first illuminator 110 illuminates a sample 3 with illumination light I.sub.1, indicated at I.sub.1d, I.sub.1d, are diffuse illumination light on the sample 3 respectively before the integrating sphere 100 is deteriorated to some extent (i.e., there is no or less possibility of measurement error concerning the reflection characteristic of the sample 3, hereinafter, this state is referred to as "initial state of the integrating sphere") and after the integrating sphere 100 is deteriorated (where a measurement error is liable to occur). Indicated at M.sub.1d, M.sub.1d ' are diffuse illumination light incident on an incident end of an optical fiber 141 respectively before and after the integrating sphere 100 is deteriorated when it is assumed that the first illuminator 110 illuminates the incident end of the optical fiber 141 with illumination light M.sub.1.
FIG. 6 shows light distribution in the integrating sphere 100 when a second illuminator 120 is driven to emit light. Assuming that the second illuminator 120 illuminates the sample 3 with illumination light I.sub.2, indicated at I.sub.2d, I.sub.2d ' are diffuse illumination light on the sample 3 respectively before and after the integrating sphere 100 is deteriorated. Indicated at M.sub.2d, M.sub.2d ' are diffuse illumination light incident on the incident end of the optical fiber 141 respectively before and after the integrating sphere 100 is deteriorated when it is assumed that the second illuminator 120 illuminates the incident end with illumination light M.sub.2.
Indicated at I.sub.2s, I.sub.2s ' are illumination components to be reflected specularly on the sample 3 as the light I.sub.2d, I.sub.2d ', hereinafter merely referred to as illumination components for specular reflection, and M.sub.2s, M.sub.2s ' are illumination components for specular reflection on the incident end as the light M.sub.d, M.sub.2d ', respectively before and after the integrating sphere 100 is deteriorated.
In this reflection characteristic measuring apparatus, it should be appreciated that the illumination light I.sub.1 that illuminates the sample 3 and the monitor light M.sub.1 that is incident on the incident end of the optical fiber 141 vary proportionally to each other. Likewise, the illumination light I.sub.2 that illuminates the sample 3 and the monitor light M.sub.2 that is incident on the incident end of the optical fiber 141 vary proportionally to each other.
The optical fiber 141 is connected to an unillustrated spectral device to indirectly monitor the illumination light I.sub.1 by monitoring change of the monitor light M.sub.1 and indirectly monitor the illumination light I.sub.2 by monitoring change of the monitor light M.sub.2. In other words, this measuring apparatus adopts the so-called "dual beam system". It should be appreciated that the measuring apparatus is so constructed as to satisfy the following equations: I.sub.1 =I.sub.1d, M.sub.1 =M.sub.1d, I.sub.2 =I.sub.2d +I.sub.2s, M.sub.2 =M.sub.2d +M.sub.2s.
In the above reflection characteristic measuring apparatus, the integrating sphere 100 is formed with a sample aperture 102 where the sample 3 to be measured is placed, illumination apertures 116, 126 respectively for allowing light to be incident from the first illuminator 110 and the second illuminator 120, and a reception aperture 131 for allowing reflected light from the sample 3 to be incident upon a receiving optical system 132. An inner wall 101a of the integrating sphere 100 is applied with a white diffuse reflection paint such as BaSO.sub.4, having a high diffusion coefficient and a high reflection coefficient.
Light from a light source 111 of the first illuminator 110 first illuminates a first region 105 having the relatively large area in the inner wall 101a of the integrating sphere 100, undergoes a multiple reflection on the inner wall 101a, and illuminates the sample 3. On the other hand, light from a light source 121 of the second illuminator 120 first illuminates a second region 104 in the inner wall 101a of the integrating sphere 100. The second illuminator 120 illuminates the sample 3 principally at an incident direction (in this case, -8.degree. direction) symmetrical to an optical axis of the receiving optical system 132 with respect to a normal axis 102a to a surface of the sample 3. .
To summarize the above, the first illuminator 110 substantially uniformly and diffusely illuminates the sample 3 and the incident end of the optical fiber 141, while the second illuminator 120 illuminating the sample 3 in such a manner that light radiated in the -8.degree. direction with respect to the normal axis 102a is strong relative to the other directions. In other words, the first illuminator 110 and the second illuminator 120 illuminate the sample 3 and the incident end of the optical fiber 141 with different light distributions.
As shown in FIG. 5, when the first illuminator 110 is driven in the initial state of the integrating sphere 100, the first illuminator 110 illuminates the sample 3 with the light l.sub.1d. Accordingly, a first reflection characteristic r.sub.1 of the sample 3 when the first illuminator 110 is driven in the initial state of the integrating sphere 100 is obtained based on spectral data of the reflected light of I.sub.1d which is inputted to the receiving optical system 132 and a sample spectral device (not shown) and a data processor (not shown).
On the other hand, as shown in FIG. 6, when the second illuminator 120 is driven in the initial state of the integrating sphere 100, the second illuminator 120 illuminates the sample 3 with the light I.sub.2d and the light I.sub.2s. Accordingly, a second reflection characteristic r.sub.2 of the sample 3 when the second illuminator 120 is driven in the initial state of the integrating sphere 100 is obtained based on spectral data of the reflected light of (I.sub.2d +I.sub.2s) which is inputted to the receiving optical system 132 and the sample spectral device and the data processor.
The data processor applies a linear combination to the thus obtained first reflection characteristic r.sub.1 and the second reflection characteristic r.sub.2 in accordance with Equation (1) to obtain a SCI reflection characteristic r.sub.i and a SCE reflection characteristic r.sub.c. EQU r.sub.1 =p.sub.1 .multidot.r.sub.1 +p.sub.2 .multidot.r.sub.2Equation 1! EQU r.sub.c 32 q.sub.1 .multidot.r.sub.1 +q.sub.2 r.sub.2
where p.sub.1, p.sub.2 are weighting factors used to obtain the SCI reflection characteristic r.sub.1, and q.sub.1, q.sub.2 are weighting factors used to obtain the SCE reflection characteristic r.sub.e, and hereinafter respectively referred to as "SCI weighting factors P.sub.1, P.sub.2 " and "SCE weighting factors q.sub.1, q.sub.2 ".
In the above reflection characteristic measuring apparatus, calibration is performed to correct a measurement error due to a deteriorated state of the inner wall 101a resulting from smear of the inner wall 101a (or simply referred to as "deteriorated state of the integrating sphere") and other factors. This deterioration cannot be avoided as the apparatus is put into a long-time use despite an attempt to continue measurement with a high precision.
Generally, calibration is performed with the use of a white reference sample before measurement. Specifically, assuming that a reflection characteristic of the white reference sample in the initial state of the integrating sphere 100 is w (the value w is known), a reflection characteristic of the white reference sample measured when the integrating sphere 100 is deteriorated is w', and a reflection characteristic of an arbitrary sample (i.e., sample 3) other than the white reference sample measured when the integrating sphere 100 is deteriorated is r', a true reflection characteristic r of the sample 3 which is supposed to be obtained in the initial state of the integrating sphere 100 is calculated in accordance with Equation (2). EQU r=(w/w').multidot.r' Equation 2!
However, the measuring apparatus in the above publication cannot properly calibrate the second reflection characteristic of the sample 3 with the use of the white reference sample when the second illuminator 120 is activated in a deteriorated state of the integrating sphere 100 because of the following reasons.
First, described is a case that the first reflection characteristic of the sample 3 is calibrated when the first illuminator 110 is activated with reference to FIG. 5. A light ray S.sub.1 incident on the receiving optical system 132 is a combination of diffused reflection components and specular reflection components of the illumination light I.sub.1 which illuminates the sample 3. Assuming that a reflection characteristic of diffused reflection on the sample 3 is r.sub.d, and a reflection characteristic of specular reflection on the sample 3 is r.sub.5, the light ray S.sub.1 can be expressed by Equation (3) because of I.sub.1 =I.sub.1d : ##EQU1## where K is a ratio of light rays that are incident on the receiving optical system 132 to a total of light rays that are reflected in the hollow space of the integrating sphere 100, and is a constant which is determined according to a configuration of the integrating sphere and the receiving optical system 132.
Since M.sub.1 =M.sub.1d, the first reflection characteristic r.sub.1 of the sample 3 is calculated in accordance with Equation (4). ##EQU2## where C.sub.1 is a proportional coefficient.
At this time, it is assumed that the light I.sub.1d and the light M.sub.1d decrease amounts of a.sub.s, a.sub.m (a.sub.s, a.sub.m &lt;&lt;1) due to a deteriorated state of the integrating sphere 100, and change to light I.sub.1d ' and light M.sub.1d ', respectively. The light I.sub.1d ' and the light M.sub.1d ' are expressed by Equation (5). EQU I.sub.1d '=I.sub.1d (1-a.sub.s) Equation 5! EQU M.sub.1d '=M.sub.1d (1-a.sub.m)
A first reflection characteristic r.sub.1 ' of the sample 3 which is measured when the first illuminator 110 is driven in a deteriorated state of the integrating sphere 100 is expressed by Equation (6). ##EQU3##
Thus, Equation (7) is calculated based on Equation (6). EQU r.sub.1 =A.sub.1 .multidot.r.sub.1 ' Equation 7!
where A.sub.1 denotes a first correction coefficient, and is expressed as A.sub.1 =(1-a.sub.m)/(1-as).
If the inner wall 101a of the integrating sphere 100 maintains the good diffusiveness, it can be assumed that a.sub.s .apprxeq.a.sub.m. Accordingly, A.sub.1 =(1-a.sub.m)/(1-a.sub.s).apprxeq.1, and r.sub.1 .apprxeq.r.sub.1 '.
On the other hand, in the case where A.sub.1 .notident.1, i.e., r.sub.1 .notident.r.sub.l ' due to a deteriorated state of the integrating sphere 100 or other causes, calibration is performed with the use of a white reference sample in accordance with Equation (2). The first correction coefficient A.sub.1 is calculated by executing the equation: A.sub.1 =w.sub.1 /w.sub.1 ' where w.sub.1 is a reflection characteristic of the white reference sample that is obtained when the first illuminator 110 is driven in the initial state of the integrating sphere 100 and w.sub.1 ' is an observed reflection characteristic of the white reference sample that is obtained when the first illuminator 110 is driven in the deteriorated state of the integrating sphere 100. In this way, the first observed sample reflection characteristic r.sub.1 ' can be properly corrected.
Next, described is a case that the second illuminator 120 is activated with reference to FIG. 6. As mentioned above, when the second illuminator 120 is driven in the initial state of the integrating sphere 100, the sample 3 is illuminated with illumination light I.sub.2 and the monitor light M.sub.2 is incident on the incident end of the optical fiber 141. This state is expressed by Equation (8). EQU I.sub.2 =I.sub.2d +I.sub.2a Equation 8! EQU M.sub.2 =M.sub.2d +M.sub.2s
Accordingly, light ray S.sub.2 incident on the receiving optical system 132 are expressed by Equation (9). As shown in Equation (9), the light ray S.sub.2 is a sum of the first term and the second term wherein the first term represents a combination of diffused reflection components and specular reflection components of the diffuse illumination light I.sub.2d and the second term represents a combination of diffused reflection components and specular reflection components of the illumination component for specular reflection I.sub.2d. As mentioned above, r.sub.d is the diffused reflection characteristic of the sample 3, r.sub.s is the specular reflection characteristic of the sample 3, and K is the ratio of the light rays S.sub.2 that is incident on the receiving optical system 132 to the total reflected rays that is radiated in the hollow space of the integrating sphere 100. EQU S.sub.2 =K.multidot.I.sub.2d (r.sub.d +r.sub.s)+I.sub.2s (K.multidot.r.sub. +r.sub.s) Equation 9!
Accordingly, the second reflection characteristic r.sub.2 is expressed by Equation (10). ##EQU4## where C.sub.2 is a proportional coefficient.
Assuming that the light I.sub.2d and the light M.sub.2d decrease an amount of a.sub.s, a.sub.m (a.sub.s, a.sub.n &lt;&lt;1) due to a deteriorated state of the integrating sphere 100, and change to the light I.sub.2d ' and the light M.sub.2d ', respectively, the light I.sub.2d ' and the light M.sub.2d ' are expressed by Equation (11). EQU I.sub.2d '=I.sub.2d (1-a.sub.s) Equation 11! EQU M.sub.2d '=M.sub.2d (1-a.sub.m)
At this time, after a multiple reflection on the inner wall 101a of the integrating sphere 100, the light I.sub.2d and the light M.sub.2d (diffuse illumination components) respectively in the illumination light I.sub.2 and the monitor light M.sub.2 illuminate the sample 3 and the incident end of the optical fiber 141. On the other hand, the light I.sub.2s and the light M.sub.2s (illumination components for specular reflection) respectively in the illumination light I.sub.2 and the monitor light M.sub.2 illuminate the sample 3 and the incident end after a single reflection on the specific region 104 of the inner wall 101a.
Accordingly, it can be said that the influence of deterioration of the inner wall of the integrating sphere 100 (i.e., lower in the reflection characteristic of the inner wall) is much smaller for the illumination component for specular reflection than for the diffused illumination component. Assuming that the lower in the reflection characteristic of the inner wall is negligible for the illumination component for specular reflection, the illumination light I.sub.2 ' and the monitor light M.sub.2 ' which vary from the initial illumination light I.sub.2 and the initial monitor light M.sub.2 as the integrating sphere 100 is getting deteriorated, are expressed by Equation (12). EQU I.sub.2 '.apprxeq.I.sub.2d (1-a.sub.s)+I.sub.2s Equation 12! EQU M.sub.2 '.apprxeq.M.sub.2d (1-a.sub.m)+M.sub.2s
Accordingly, an observed second sample reflection characteristic r.sub.2 ' of the sample 3 which is measured in a deteriorated state of the integrating sphere 100 is expressed by Equation (13). ##EQU5##
As shown in Equations (10) and (13), the second true reflection characteristic r.sub.2 and the second observed sample reflection characteristic r.sub.2 ' before and after the integrating sphere 100 is deteriorated cannot be expressed with a simple proportional relationship, whereas the first true reflection characteristic r.sub.1 and the first observed sample reflection characteristic r.sub.1 ' before and after the integrating sphere 100 is deteriorated are expressed with a simple proportional relationship, as shown in Equation (7). Accordingly, the second observed reflection characteristic r.sub.2 ' of the sample 3 cannot be properly calibrated with the use of the white reference sample in accordance with Equation (2) because of the fact that the deteriorated state of the integrating sphere 100 gives different influences to the diffused illumination component and the illumination component for specular reflection as mentioned above.
Specifically, the light intensity of the diffused illumination component relative to the illumination component for specular reflection on the sample 3 and the incident end of the optical fiber 141 (I.sub.2d /I.sub.2s of the illumination light I.sub.2 and M.sub.2d /M.sub.2s of the monitor light M.sub.2) unavoidably changes when the second illuminator 120 is driven as the integrating sphere 100 is getting deteriorated.
However, in the reflection characteristic measuring apparatus disclosed in Japanese Unexamined Patent Publication No. 9-61243, the weighting factors p.sub.1, p.sub.2, q.sub.1, q.sub.2 in Equation (1) are set on the assumption that the above relative light intensities do not change.
In the case where the relative light intensity is changed to a non-negligible extent due to a deteriorated state of the integrating sphere 100, it is required to calculate the weighting factors again. Calculation of the weighting factors requires measurements of reflection characteristics of plurality of reference samples as mentioned in the publication. Further, it requires ample care to store these reference samples in a stable condition free from change of the reflection characteristic.
Accordingly, there has been demanded a simple calibration manner to correct a second observed sample reflection characteristic r.sub.2 ' after the integrating sphere is deteriorated which makes it possible to use weighting factors obtained before a deterioration of the integrating, sphere as the simple calibration for first sample reflection characteristic r.sub.1 ' using a white reference sample.
Further, there have to be considered other factors which may lead to a measurement error when measuring the first reflection characteristic and the second reflection characteristic using the first illuminator 110 and the second illuminator 120 besides deterioration of the integrating sphere 100, for example, deterioration of optical elements other than the integrating sphere 100, such as the receiving optical system 132, and changes in the ambient temperature and humidity. Accordingly, there has also been demanded a measure that can compensate the measurement error of the first reflection characteristic and the second reflection characteristic resulting from these factors.